1. Field of the Invention
Aspects of the present invention relate to a cap assembly and a secondary battery having the same, and more particularly, to a cap assembly that can prevent current from flowing after breakage of a vent member caused by an internal pressure of the secondary battery greater than a predetermined level by preventing the broken vent member from contacting a cap.
2. Description of the Related Art
With recent development and production of portable electronic appliances having a compact, lightweight size, such as cellular phones, notebook computers. and video cameras, the electronic appliances require battery packs so as to operate in any place where power is not supplied. The battery pack, or rechargeable or secondary battery, is made of nickel-cadmium (Ni—Cd), nickel-metal hydride (Ni-MH), or lithium (Li).
Specifically, the Li secondary battery has been widely used to power the portable electronic appliances as the Li secondary battery has an operating voltage on the order of a three times higher than the Ni—Cd or Ni-MH batteries. Further, the Li second battery has a higher energy density per unit weight than the Ni—Cd or Ni-MH batteries. The Li secondary battery is classified according to a type of electrolyte used, such as a Li ion battery, which uses a liquid electrolyte, and a Li polymer battery, which uses a polymer electrolyte. The Li secondary battery is also classified according to the shape of the battery, such as cylindrical, rectangular, and pouch-shaped batteries.
FIG. 1 is a cross-sectional view of a conventional secondary battery. Referring to FIG. 1, the conventional secondary battery includes an electrode assembly 10; a can 20 (e.g., a container) having an opened top and containing an electrolyte (not illustrated), which enables Li ions to move between the members of the electrode assembly 10; and a cap assembly 30 to seal the can 20. The cap assembly 30 is electrically connected to the electrode assembly 10. As shown in FIG. 1, the electrode assembly 10 includes: a positive electrode plate 11 having a positive electrode tap connected to a positive electrode collector (not illustrated) to which a positive electrode active material (not illustrated) is applied; a negative electrode plate 12 having a negative electrode tap connected to a negative electrode collector (not illustrated) to which a negative electrode active material (not illustrated) is applied; and a separator 13 disposed between the positive electrode plate 11 and the negative electrode plate 12 for insulating the two electrode plates 11 and 12 from each other. An electrode tap 14 may be connected from one of the positive electrode plate 11 and the negative electrode plate 12 to the cap assembly 30 disposed on the can 20 as shown in FIG. 1. In addition, another electrode tap (not shown) may be connected from the other of the positive electrode plate 11 and the negative electrode plate 12 to the can 20.
The positive electrode active material may include a transition metal oxide with a Li ion or a Li chalcogenide compound, for example, a metal oxide, such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4 or LiNi1-x-yCoxMyO2 (here, 0≦x≦1, 0≦y≦1, 0≦x+y≦1, M is aluminum (Al), strontium (Sr), magnesium (Mg), lanthanum (La), etc.), and the negative electrode active material may include crystalline carbon, amorphous carbon, carbon complex, carbon material such as a carbon fiber, carbon nanomaterials such as carbon nanotubes, Li metal, or Li alloy.
The positive or negative electrode collector may be formed of one selected from the group consisting of stainless steel, nickel (Ni), copper (Cu), aluminum (Al) and an alloy thereof. Generally, the positive electrode collector is formed of Al or an Al alloy, and the negative electrode collector is formed of Cu or a Cu alloy, as such formation increases the efficiency of the battery.
The separator 13 is disposed between the positive electrode plate 11 and the negative electrode plate 12 to prevent a short circuit. The separator 13 further allows Li ions to move therethrough between the positive electrode plate 11 and the negative electrode plate 12. The separator 13 may be formed of a polyolefin-based polymer layer such as polyethylene (PE) or polypropylene (PP), or a multi-layer thereof.
The can 20 may be a container formed of a metallic material having an opened top and a cylindrical shape. Alternatively, the can 20 may be formed in a rectangular or pouch shape. The can 20 may be formed of a metallic material that is lightweight, flexible, and functional as a terminal, for example, Al, an Al alloy, or stainless steel.
The cap assembly 30 includes a cap 31 coupled with the top opening of the can 20 to seal the can 20 and connected to an external terminal (not shown); a vent member 33 which breaks to exhaust a gas when an internal pressure of the secondary battery is greater than a certain level due to the gas generated from the electrode assembly; a bottom cap 34 connected to the electrode tap 14; an insulating plate 36 to prevent unnecessary short-circuit between the bottom cap 34 and the vent member 33; and a gasket 37 to insulate the can assembly 30 from the can 20. A positive temperature coefficient (PTC) material 32 may be disposed between the cap 31 and the vent member 33 so as to improve the stability and reliability of the secondary battery, and a sub plate 35 may be disposed below the bottom cap 34.
In the cap assembly 30 of the conventional secondary battery, when the internal pressure of the secondary battery reaches a certain level caused by gas generated from the electrode assembly 10, the vent member 33 is broken to prevent current flowing from the battery and to exhaust the gas.
However, in the above-described cap assembly 30, if the broken vent member 33 is in contact with the cap 31, an arc may occur between the vent member 33 and the bottom cap 34 or the sub plate 35, which is disposed therebelow, such that current flows from the battery, which results in a decrease in reliability of the secondary battery.